1. Field of the Invention
This invention relates to a process for renewing the adsorptive capacity of a bed of carbon by electrochemical adsorbate transformation. By this process, a bed of active carbon is contacted with an effluent containing at least one hydroxyaromatic compound containing at least one OH group joined directly to the aromatic ring until an appreciable amount of the compound is adsorbed onto the bed of carbon. A conductive solution is then introduced to the bed of carbon and the carbon is made the positive potential of an electrolytic cell. A positive potential is applied to the bed of carbon which is sufficient enough to transform the compound into a species which has either a high affinity for the carbon or a low affinity for the conducting solution, with the potential being less than that required for complete oxidation of the compound. The applied potential is then withdrawn and effluent containing at least one hydroxyaromatic compound is reintroduced to the bed of carbon. This procedure is repeated until the bed of carbon is no longer able to remove additional compounds from the effluent.
2. Description of the Prior Art
The treatment of effluents, such as waste water, is a major consideration of all factories, power plants and municipalities. Today, there is hardly an industry in existence which doesn't have to allocate a substantial amount of money to regenerate or recondition effluents used for washing, cooling or cleaning purposes in some phase of a production process. Aside from the major costs associated with designing and constructing treatment facilities, and purchasing expensive carbon regeneration equipment, there is the cost of purchasing initial carbon fill and replenishing the five to fifteen percent which is used every time a conventional regeneration is accomplished. Any method which could increase the adsorption ability of the carbon thereby extending the time period between conventional regenerations would yield a reduction of the amount of carbon which would have to be used. This will decrease the daily replacement cost of the carbon and could also possibly reduce the cost of the regeneration equipment.
Most pollutants in aqueous effluents contain hydroxyaromatic compounds, which compounds are defined for purposes of this invention as compounds containing at least one OH group joined directly to the aromatic ring. Examples of such hydroxyaromatic compounds are phenol, substituted phenol, naphthol, etc. These compounds are highly soluble in water and therefore do not tend to load up a carbon bed as heavily as other components contained in the effluent. Therefore, any method which would increase the total uptake of such compounds per weight of carbon employed would increase the efficiency of the carbon treating process. Presently there are two noticeable ways to renew the adsorptive capacity of a bed of carbon, by oxidation and by adsorption-desorption. Oxidation involves a process in which the carbon is physically transferred by mechanical means from the bed to a furnace where it is burned. In the furnace about ten to twenty percent of the carbon is burned up in trying to sufficiently oxidize the carbon. The remaining carbon is then returned to the bed where adsorption again takes place. The second method encompasses adsorbing the diluents from an effluent onto the surface of the carbon. The remaining effluent is then withdrawn and replaced by a second smaller solution. A potential is then applied to the bed of carbon to get the diluents to desorb into the second more concentrated solution. When the second solution acquires a sufficient concentration so that it will no longer function as a source which will extract additional diluents from the carbon, it is withdrawn and disposed of. The carbon is regenerated by this desorbing step, at least to a certain extent, and can now absorb additional diluents contained in another effluent stream. The major deficiencies of this system are that the second somewhat concentrated solution has a very large volume and has to be disposed of, such as by dumping which is an environmental hazard and very large quantities of an expensive electrolytic salt are required to be present in the second solution to accomplish desorption of the diluent from the carbon.
Various patents describe the electrochemical techniques of treating effluents. For example, Tarjanyi, et al., U.S. Pat. No. 3,730,864 entitled: "Descreasing the Phenolic Content of Liquids by an Electrochemical Technique", is directed to a method for decreasing the phenolic content of a solution which comprises passing an electric current through a solution containing the phenolic material and oxidizing it completely to carbon dioxide and water. This solution is contained as the electrolyte in a cell having at least one positive and one negative electrode between which the current is passed. The electrode also contains a bed of particles which are distributed therein such that the porosity of the bed is from about 40 to 80 percent. The electrolysis of the electrolyte is continued until the desired reduction in the phenolic content is obtained.
A second patent is Makrides, et al. U.S. Pat. No. 3,730,855 entitled: "Electrochemical Control of Adsorption and Desorption With Activated Carbon", is directed to a method and apparatus for controlling the adsorption-desorption behavior of activated carbon. Adsorption and desorption are controlled by controlling the potential at the interface between the carbon and the solution containing the species to be adsorbed or the solution into which the adsorbed species are to be desorbed. Makrides indicates a technique whereby organic materials are adsorbed onto a bed of carbon when an electrical potential is maintained across the interface of the carbon and the solution. After adsorbing for a period of time the carbon will become saturated with the organic material and will be unable to adsorb as efficiently. When this happens, the carbon is contacted with a second solution, a conducting solution, and the potential value is changed to either a negative potential or a higher positive potential. By maintaining the solution at this new potential value, the previously adsorbed organic material will desorb into the second solution. After the carbon is sufficiently desorbed (regenerated), it is again exposed to another effluent stream and will again adsorb the organic material. This adsorbing-desorbing procedure is continuously repeated provided a potential difference is maintained. Although Makrides discloses that a wide variety of organic materials can be used, such as: organic acids and their salts, amines, phenols, proteins, sugars, milk solids, urea, and anions such as PO.sub.4 -3 and NO.sub.3 -1, only sodium chloride and acetic acid, sodium chloride and amyl alcohol, and Na.sub.2 SO.sub.4 are exemplified. There is no appreciation that hydroxyaromatic compounds containing at least one OH group joined directly to the aromatic ring cannot be desorbed at positive potential.
Now a new process for renewing the adsorptive capacity of a bed of carbon by electromechanical adsorbate transformation has been invented. This process is especially useful for hydroxyaromatic compounds and is particularly useful in the standard tertiary water treatment processes. The main advantage of the process over other electro-oxidative methods for water purification is its low power requirement. This electric power saving is mainly due to avoidance of complete oxidation of the hydroxyaromatic adsorbate; e.g., adsorbed phenol evidently requires only two electrons per molecule to change it to a species which has either a strong affinity for the carbon or a low affinity for the conducting solution, while twenty-eight electrons per molecule would be required for complete oxidation to carbon dioxide and water. Power costs are also lowered by low total voltage and the effect of low current on resistive losses, leading to a total decrease in power of some twenty-fold compared to conventional electro-oxidative water treatment processes.
An object of this invention is to provide a process for renewing the adsorptive capacity of a bed of carbon to adsorb hydroxyaromatic compounds. A more particular object of this invention is to provide a process for renewing the adsorptive capacity of a bed of carbon so that a substantial savings in carbon replacement costs, capital investment, and operational costs can be realized when this process is incorporated into a standard tertiary water treatment process.
A further object of the present invention is to provide a process that will lead to approximately a twenty-fold decrease in power costs compared to the conventional electromechanical oxidation of organic compounds.
Still further, an object of the present invention is to provide a process which will decrease the required volume of conducting (renewal) solution as much as 10,000 fold, as compared to a process which renews carbon capacity by desorption.
Other objects of this invention will become apparent to one skilled in the art based upon the ensuing description.